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The analysis and extraction of the appropriate signal's features in structural health monitoring applications is one of the major challenges on which the robustness of the designed systems relies. Many strategies have been developed in the past, which utilise the identification of amplitude-based parameters for the evaluation of structural integrity. However, these parameters usually require a baseline reference, which might be extensively affected by noise, environmental or mounting conditions. This paper illustrates the applicability of Hilbert transform and Hilbert-Huang transform on the postprocessing of guided ultrasonic waves for evaluating the condition of relatively complex structural health monitoring applications. Two case studies are presented to demonstrate the suitability of the techniques, namely, the damage monitoring of an aluminium repaired panel and the cure level monitoring of symmetric carbon fibre-epoxy composite laminates. In the first case study, the technique exhibits sensitivity in propagation paths within damaged areas and shows good agreement with the developed damage, enabling the identification of critical areas. In the second study, the technique demonstrates a significant advantage over the traditionally adopted approaches and predicts accurately the cure level of the polymeric composite system. Inspection with guided ultrasonic waves is a wave-propagation-based method, which has lately attracted the attention of the scientific community, offering various industrial applications in monitoring pipes, pipelines and rails [7]. Guided waves propagate in a bounded media, which interacts with the boundaries of the structure and propagate through its thickness in two possible modes; symmetric (S n ) and antisymmetric (A n ). The mathematical equations that describe their behaviour are the same with bulky waves. The only difference is that, in the case of guided waves, the equations should satisfy the boundary conditions. The most widely used technique for health monitoring is inspection with Lamb waves. Lamb waves are elastic perturbations that occur in solid plates with free boundaries. Those perturbations are a combination of displacements that occur both in the direction of wave propagation and perpendicularly to the plane of the plate [8]. Lamb waves have been extensively used for health monitoring of advanced, complex structures. The scope of the current investigation concentrates on aspects of Lamb wave's application for health monitoring of two case studies, the first on damage monitoring of bonded patch repairs and the second on monitoring the cure process of a composite laminate.Adhesively bonded repair patches have lately received a considerable amount of interest for their potential repair applications for temporary (emergency or field) solutions to structural damage. In the aerospace industry, the interest in bonded patches emerges from the need to increase the operational life of ageing aircrafts. Such aircrafts are estimated to be approximately 30% of the worldwide fleet [9]....

This work focuses on structural health monitoring aspects of composite adhesively bonded repairs, evaluating their performance with guided ultrasonic waves. These repairs have shown remarkable potential in addressing repairability demands in new composite aircraft. More specifically, the behavior of a scarf repair under axial tensile loading was monitored with guided ultrasonic waves. The signal post-processing techniques focused on the extraction of the appropriate features, on the application of the pattern recognition and dimension reduction algorithms and on their subsequent correlation with the damage. A principal component analysis was employed that operated as a benchmark for the proposal of a more advanced data reduction method, the nonlinear principal component analysis. Appropriate damage indices were extracted and the results were correlated with images taken through a digital image correlation technique. The correlation of the extracted features with the early stage damage was performed and conclusions about the recovered strength through the scarf repair were deduced. The study focused on the selection of appropriate signal features and on their subsequent investigation through an outlier analysis. The limits of the applied outlier analysis were interpreted through principal component analysis and optimized through the concept of principal curves as derived through the nonlinear principal component analysis.

The present work focuses on the structural health monitoring of an aluminium vertical helicopter stabiliser with a pre-introduced crack which was repaired with an adhesively bonded composite patch. The structure was monitored under bending fatigue and its performance was evaluated with Lamb waves, lock-in thermography and ultrasonic testing. Outlier analysis of Lamb waves captured the onset and progress of the damage in the form of patch debonding, enabling the identification of five damage-severity regions. Principal component analysis showed distinctive clusters that corresponded to different damage levels while the application of principal curves on the selected features proved to be an additional damage detection index. Amplitude and phase lock-in images accurately captured the onset and evolution of the damage in the form of patch debonding and honeycomb/skin debonding in agreement with the damage indices obtained from Lamb waves. C-scan further validated the damage mechanisms that were captured by the other methods.

Life extension of aging aircraft with adhesively bonded repair patches has received significant interest over the last 10-15 years. This repair method has several advantages over riveted patches since it introduces lower stresses in the repaired area and reduces weight. It is a promising repair technique, which can be also applied to composites, where the load transfer paths through mechanical fasteners can cause local damage. However, patch debonding might occur when the structure is loaded, and the damage might continue propagating in the substrate. As a result, a robust structural health monitoring system needs to be designed and implemented in order to continuously assess the integrity of the repaired area. This paper illustrates the nondestructive inspection with Lamb waves of a repaired aluminium flat plate with a bonded external patch. The damage developed was assessed with outlier analysis, which is a pattern recognition method.